The present invention relates to an operational amplifier having a function for protecting a short-circuit of the output terminal.
An ECU (Electrical Control Unit) is installed in a vehicle and electrically connected to loads provided in various portions of the vehicle and to other ECUs provided in the vehicle by means of harnesses.
A conventional connection topology of a vehicle uses the body as ground so that signal harnesses including driving harnesses are often independently wired so that, short-circuit failures, as failures in the course of wiring the vehicle, mainly occur between the signal harnesses and the body.
In order to avoid the short-circuit failures, an operational amplifier used in the output unit of the ECU additionally includes a protecting circuit for protecting the short-circuit failure between the output terminal of the output unit and the ground.
FIG. 3 shows an electrical structure of a driving unit and an output unit of a conventional operational amplifier. In FIG. 3, a transistor Q1 and Q2, which constitute a push-pull output circuit 3, are connected in series through an output terminal 4 between the power and ground lines 1 and 2. A resistor R1 is connected between the transistor Q1 and the output terminal 4. Transistors Q3 to Q6 constitute a driving circuit 5 for driving the transistors Q1 and Q2. A transistor Q7 and the resistor R1 are served as a protecting circuit 6. That is, the transistor Q7 turns on when the output terminal 4 is short-circuited to the ground line 2, causing the transistor Q1 to turn off.
By the way, each of recent vehicles comprises many ECUs to obtain multifunction so that the harnesses installed in the vehicle for connecting the many ECUs are increased, thereby causing the weight of the harnesses to be increased.
In order to decrease the number of harnesses, some of the vehicles may have the connection topology of the power harness in that the single power harness from the battery is bundled together with the signal harnesses and the single power harness and the single harness is sequentially cascade-connected to each of the ECUs in place of the connection topology in that power harnesses from the battery are parallelly connected to the ECUs, respectively.
In addition, in order to prevent foreign noises from entering into the harnesses, in many cases, the signal harnesses, the power harnesses and the ground harnesses are wired together, which is served as a guard ring. This connection topology, however, causes the signal harnesses and the power harnesses to be short-circuited, in addition to the short-circuit failures of the signal harnesses and the body of the vehicle.
The operational amplifier shown in FIG. 3, even if the output terminal 4 is short-circuited to the power harness that has the same potential as the power line 1, is not immediately broken down. If the short-circuit state is kept, however, the operational amplifier may cause thermal breakdown.
In order to counter the problem, providing the protecting circuit which is the same as the protecting circuit 6 in the operational amplifier in FIG. 3 for making turn off the transistor Q2 is considered.
The structure, however, the register inserted between the output terminal 4 and the transistor Q2 makes increase the minimum voltage that the operational amplifier can output, whereby the operational amplifier having the structure may not be used in cases where loads that do not correspond to the voltage region that the operational amplifier can output.
Then, in order to count the problem, conventional amplifiers need to comprise another protecting circuit for detecting the occurrence of the short-circuit failure to electrically disconnect the output terminal, or to make large each size of each element (component) of the operational amplifier to increase the capability of each element of resisting the thermal breakdown, causing the whole circuit size of the operational amplifier to increase and the cost thereof to increase.